Experimental Study of Flow Field and Transition Characteristics in Rod Bundle Channel

•PIV used to study flow fields & turbulence in 5x5 rod bundle across 22 cases (Reynolds: 310-12296).•Higher turbulence intensity at lower Reynolds due to significant relative velocity gradient in rod bundle.•Transition Reynolds ≈ 900 in rod bundle, less distinct than in pipes.•Fluctuation velocity decreases with Reynolds, abrupt rise at transition Reynolds.•Non-uniform transition Reynolds leads to varying turbulence intensity in rod bundle.